Port d303f4fb https://crrev.com/c/2081816

Original Commit Message:

  In the past we've used the isolate argument to signal whether we were
  in unicode mode (nullptr) or not (the real isolate). This is no longer
  needed, and in fact breaks no-i18n mode which always expects to have a
  real isolate.

Change-Id: I90a69a38cb4e74da46b3030843bc84d6d53571c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2082932
Auto-Submit: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66549}